1. Field of the Invention
The present invention relates to technology for diagnosing the presence of faults (fuel vapor leaks) in a fuel vapor treatment unit of an internal combustion engine, and in particular to technology for preventing erroneous diagnosis and increasing diagnosis accuracy.
2. Description of the Related Art
With conventional fuel vapor treatment units for internal combustion engines, diffusion of fuel vapor into the atmosphere is prevented by temporarily adsorbing fuel vapor generated in the fuel tank etc. into an adsorption unit (canister), and then, under predetermined engine operating conditions, de-adsorbing the adsorbed fuel vapor and mixing this with purge air, and drawing the purge mixture into an engine intake system, while controlling the flow of purge mixture with a purge control valve (refer to Japanese Unexamined Patent Publication No. 5-215020).
With the above units however, if a crack occurs along the fuel vapor piping, or a fault occurs in a seal at a fuel vapor piping connection, then the fuel vapor will diffuse into the atmosphere from the leak portion, so that the original diffusion prevention effect cannot be fully realized.
For an apparatus for diagnosing the presence of faults (fuel vapor leaks) in the fuel vapor treatment unit, the following methods have been contemplated.
That is to say, fault diagnosis of a fuel vapor treatment unit is carried out by closing a purge control valve disposed in a purge passage between a canister and an intake passage at the time of diagnosis, being after the engine has stopped or while the vehicle is travelling, then supplying air which is pressurized by a pump device provided for diagnosis, to inside a sealed fuel vapor supply system on the downstream side of the purge control valve, and making a judgement of a pressure rise of the fuel vapor supply system, or a time until a predetermined pressure is reached.
After the engine has stopped however, the quantity of vapor generated inside the fuel tank due to the heat from the engine is comparatively large, and hence this is an unsuitable condition for accurate fault diagnosis. Moreover, when the vehicle is travelling, in addition to the engine heat, there is the likelihood during fault diagnosis of influences due to changes in atmospheric pressure from travelling along roads of different altitudes. Furthermore, the liquid level inside the fuel tank also is agitated due to vibrations from the engine or the road surface, thus becoming a source of erroneous diagnosis.
The present invention takes into consideration such heretofore problems, with the object of being able to prevent erroneous diagnosis in a fuel vapor treatment unit, and thus carry out accurate diagnosis.
Furthermore, without requiring modification of the hardware for fault diagnosis itself, it is an object to improve diagnosis accuracy without increasing cost, by making the timing for executing fault diagnosis a suitable timing.
Moreover, it is an object to speed up execution of fault diagnosis and thus avoid beforehand, adverse affects on handling due to faults.